Field of the Invention
Unprecedented control of metal surface accessibility is facilitated by synthetic pockets associated with ligands bound to a metal. More particularly, a metal cluster core, e.g., an Ir core, with ligands, e.g., three calixarene phosphine ligands, bound thereto, can have binding sites accessible to one species, e.g., CO, but not another, ethylene. Such metal clusters can find application in catalysis and gas-phase separations.
Description of the Related Art
Enzymes have evolved to incorporate active sites within pockets that exhibit exquisite levels of specificity to reacting substrate molecules. (O. Khersonsky, D. S. Tawfik, Annu. Rev. Biochem. 79, 471-505 (2010)). Preferential substrate access and binding to active sites lead to significantly higher catalytic proficiencies for native substrate molecules, an observation that guided early understanding of accessibility in enzymes based on the well-known lock-and-key analogy, as well as the design of catalytic antibodies. (P. G. Schultz, R. A. Lerner, Science 269, 1835-42 (1995)) Preferential accessibility in these biological systems is thought to be controlled by the molecular-level structure and composition of the pocket enveloping the active site. (P. J. O'Brien, D. Herschlag, Biochemistry 40, 5691-9 (2001); L. Quintanar et al., J. Am. Chem. Soc. 127, 13832-45 (2005)). However, the features that control preferential accessibility are so subtle that, until now, attempts to create synthetic molecular pockets have fallen far short of replicating the degree of preferential binding and activation of reacting substrates found within biological systems.
In catalysis by synthetic materials, a high degree of shape selectivity is known to occur within the confines of interior micropores of zeolite crystals, (Y. Roman-Leshkov, M. Moliner, M. E. Davis, Chem. Mater. 22, 2646-2652 (2010); C. W. Jones, M. Tsapatsis, T. Okubo, M. E. Davis, Micropor. Mesopor. Mat. 42, 21-35 (2001); C. B. Khouw, M. E. Davis, ACS Sym. Ser. 517, 206-221 (1993)) but this often carries undesirable consequences of mass-transport limitations and limits the size of reactants and products to those that are small enough to access the interior active sites. Metal clusters encapsulated within a bulk microporous zeolitic framework are known to exhibit shape-selective binding due to the size discrimination characteristics of the zeolite (see Goel et al. J. Am. Chem. Soc. 2012, 134, 17688-17695). This is significantly different from the current invention in which selection of a fluid species arises due to the spatial arrangement and local environment created by ligands directly bound to the metal surface, rather than the microporosity of an encapsulating bulk framework. Another type of less well-developed shape selectivity in synthetic active sites, the nest effect, (T. Degnan, J. Catal. 216, 32-46 (2003)), relates to shape selectivity of an active site that is located at the terminus of a micropore on the external surface. In contrast to shape selectivity imposed within the interior of a zeolite catalyst, the nest effect is the closest analogy to shape-selective catalysis in enzymes, because of shape-selectivity being induced via active-site location within a surface pocket rather than a three-dimensional cavity of a bulk material. A nest effect has been used to explain shape selectivity on the external surfaces of zeolites, (T. F. Degnan, C. M. Smith, C. R. Venkat, Appl. Catal. A-Gen. 221, 283-294 (2001); A. Corma et al., Micropor. Mesopor. Mat. 38, 301-309 (2000)) and in surface-imprinting strategies. (C. P. Canlas et al., Nature Chem., 4, 1030-1036 (2012), G. Wulff, B. Heide, G. Helfineier, React. Polym. 6, 299-310 (1987)). However, to-date, selectivities achieved when using the nest effect have been modest, even for molecules of significantly differing size, and do not approach the selectivity that would be useful to industry for applications such as separations involving a multicomponent fluid mixture and sensing. Such a separation typically involves adsorbing one or more components from a fluid mixture preferentially over others within the same mixture. The fluid may be either gas phase or liquid phase. A sensing application would involve the preferential adsorption of one or more components from a fluid mixture, which would be used to determine the presence and/or relative amounts of these component(s) in the mixture.